The invention relates generally to a spot size converter for coupling an optical fiber and optical waveguide integrated devices, and a method of fabricating the same. More particularly, the invention relates to a spot size converter capable of significantly reducing the coupling loss and the reflection loss between an optical fiber and optical waveguide integrated devices, and a method of fabricating the same.
Conventional technologies relating to a method of fabricating this type of spot size converter typically use well-known butt-joint integration method.
The butt-joint integration method is an integration technology which positions an active-type optical waveguide and a passive-type optical waveguide on the same plane and connects of the cross sections of the two optical waveguides. This method is described in an article of E. Gaumont, xe2x80x9cButt coupling process for InP based photonic integrated circuit,xe2x80x9d Proc. 8th Int. Conf. of InP and related materials, p256. The above integration technology has an advantage in that it achieves a high integration degree since two optical waveguides are positioned at the same layer and it does not require a tapered structure. As the technology directly couples an active-type optical waveguide and a passive-type optical waveguide, however, there is a disadvantage that scattering loss and reflection problem tend to occur due to defects at the cross-section boundary.
In order to solve the problem of scattering loss and the reflection problem at the boundary of the butt-joint integration method, a double-layer optical waveguide integration method has recently been proposed. The double-layer optical waveguide integration method is described in an article of J. Y. Emery, xe2x80x9cHigh performance 1.55 xcexcm polarization-insensitive semiconductor optical amplifier based on low-tensile-strained bulk InGaAsP,xe2x80x9d Electronics Letters, Vol.33, p1038, 1997. The double-layer optical waveguide integration method vertically positions a passive-type optical waveguide and an active-type optical waveguide but requires a high degree of precision and repeatability in the fabricating process.
It is therefore an object of the present invention to provide a spot size converter which has a low coupling loss and reflection loss and a method for fabricating such spot size converter with ease and at a low manufacturing cost.
In order to accomplish the above object, the present invention provides a spot size converter for converting the spot size of input optical signal received from an input optical fiber and outputting the converted optical signal, said spot size converter comprising an input optical waveguide formed on a semiconductor substrate and connected to the input optical fiber to receive input optical signal; an output optical waveguide formed on the input optical waveguide to form a double-layer structure with the input optical waveguide on the semiconductor substrate and to output the optical signal after converting the spot size of the optical signal; and a unidirectional side-tapered optical waveguide formed in the same layer as the output optical waveguide and integrated to one side of the output optical waveguide facing the input optical fiber, and having a shape tapered in one side along a given length in the longitudinal direction of the output optical waveguide, wherein the spot size of the input optical signal transferred to said unidirectional side-tapered optical waveguide is gradually converted while the optical signal travels through the unidirectional side-tapered optical waveguide toward the output optical waveguide due to the continuous change in the effective reflective index.
In another aspect of the invention, the invention provides a method of fabricating a spot size converter comprising the steps of sequentially forming a first semiconductor layer for forming an input optical waveguide, and a second semiconductor layer for forming a unidirectional side-tapered optical waveguide and an output optical waveguide on a semiconductor substrate; etching the second semiconductor layer so that the width of said second semiconductor layer corresponds to a desired width of the input optical waveguide; forming a mask on said second semiconductor layer, the mask having a width corresponding to a desired width of the output optical waveguide and having a tapered portion on one end, the tapered portion having a shape one side of which is tapered along a given length in the longitudinal direction of said output optical waveguide to correspond to a desired shape of the unidirectional side-tapered optical waveguide; and respectively etching the second semiconductor layer and the first semiconductor layer using the mask and the second semiconductor layer as a mask.